This invention relates to an apparatus for making an optical fiber preform which can be drawn into a fiber with an improved core/clad concentricity.
In the manufacture of an optical fiber, a preform is typically made using either outside vapor deposition (OVD), vapor axial deposition (VAD) or modified chemical vapor deposition (MCVD) techniques. OVD and VAD preforms are then dehydrated and consolidated to form a solid glass blank while MCVD preforms are collapsed to form a solid glass blank. The glass blanks are then drawn into fiber. Alternatively, the glass blanks can be drawn into core cane which is then overclad using OVD or rod-in-tube techniques to form an overclad preform. The overclad preform is subsequently dehydrated and consolidated to form an overclad glass blank which is then drawn into fiber.
One parameter of interest in the drawn fiber is the core/clad concentricity (hereinafter referred to as concentricity). Concentricity is a measure of how well the core of a fiber is centered with respect to the overall center of the fiber. FIG. 7 shows a cross-section of an optical fiber 70 with core region 71 having center A and clad region 72 having center B. The concentricity can then be characterized by the distance X between core center A and clad center B.
It is believed that for an overclad preform, the concentricity is determined during the overclad soot deposition step. FIG. 1 depicts a cane assembly 1 in which a rod 2 is attached to an upper cane handle 3 and a lower cane handle 4. If a cane assembly as shown in FIG. 1 is not made perfectly straight, it will wobble when rotated during the soot deposition process. This wobble will result in uneven deposition of soot around the cane due to the relative movement of the cane assembly with respect to the soot stream. Even if a cane assembly could be made perfectly straight, wobble is likely to develop because of changes in stresses in the cane assembly which result from heating the cane assembly during the soot deposition process.
To provide higher throughput and equipment utilization, the size (both length and diameter) of an optical fiber preform has been increased in recent years. As the length is increased, the potential for problems with wobble is substantially increased, resulting either from cane assemblies which are not perfectly straight or from changes in stresses in the cane assembly during the soot deposition process.
FIG. 2 depicts a typical apparatus used for overclad soot deposition onto a cane assembly. Cane assembly 10 is placed in upper chuck 11. Lower chuck 12 is attached to the lower end of cane assembly 10. Upper chuck 11 is attached to upper spindle 13. Upper spindle 13 is also attached to spindle motor 14 which is in turn attached to weighing device 15. Lower chuck 12 is attached to lower spindle 16. Spindle motor 14 is used to rotate upper spindle 13, upper chuck 11, cane assembly 10, lower chuck 12 and lower spindle 16, thereby flowing soot deposition to occur about the outer surface of cane assembly 10. Burner 21 represents a well known burner used in OVD processes, such as that disclosed in Backer et at. U.S. Pat. No. 5,067,975, the pertinent potions of which are incorporated herein by reference. Air bearings 17, 18, 19 and 20 are essentially frictionless in the direction of the axis of cane assembly 10. This allows the weight of the cane assembly to be accurately determined by weighing device 15 during the soot deposition process. By taring, or zeroing, the initial weight of the cane assembly and components of the apparatus prior to starting the soot deposition process, the weight of soot deposited on cane assembly 10 can be accurately determined. This allows a control system (not shown) to determine, based on the weight of soot deposited, when to change process parameters, including the composition of the soot being deposited, and when to shut down the soot deposition process upon completion thereof.
Upper chuck 11 and lower chuck 12 can be adjusted using screws 22 to exert moments on the ends of the cane assembly to reduce any wobble which might result from untrueness about the centerline of the chucks. The process for adjusting the upper and lower chucks is called truing and can be tedious and time consuming. Additionally, the moments exerted on the ends of the cane assembly generate stresses within the cane assembly. The subsequent heating of the cane assembly during the soot deposition process will change the stress pattern within the cane and will result in wobble of the cane assembly during soot deposition.
It is an object of the present invention to improve the core/clad concentricity of an optical fiber by reducing the wobble during overclad soot deposition. It is another object of the present invention to improve the utilization of the apparatus used for overclad soot deposition.